Enable feature flag ci_cross_pipeline_artifacts_download

What does this MR do?

Related to #255983 (closed)

Related to #287622 (closed)

This MR enables by default the feature flag ci_cross_pipeline_artifacts_download and adds documentation for the feature that allows a job in a parent-child pipeline to download artifacts from a pipeline in the same parent-child pipeline hierarchy.

This MR describes how the feature works.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team